Polylactic acid (PLA) is a biodegradable and recyclable thermoplastic produced from renewable resources such as plant starch [78] . It is a semi-crystalline thermoplastic with an extrusion temperature of 160˚C - 220˚C. PLA is well-known as an easy material to deal with during 3D printing. It is considered as an environmentally friendly material, biodegradable and biocompatible, with good processability and low-cost production. It can be printed at high speeds and faces fewer shrinkage issues as compared to other materials. With the FDM technique, a hot printing bed is not required for PLA [86] . However, PLA is susceptible to degradation during its use, melting, and the recycling processes [87] [88] [89] . The thermal, mechanical, and fracture behaviors of PLA processed by extrusion were investigated by Nascimento et al. [90] . They concluded that a single recycling process resulted in only a few structural changes rather than altering significantly material performance. In contrast, repeated processing cycles resulted in the degradation of PLA. The chain scissions occurred after seven processing cycles. Reduced molecular weight led to a decrease in stress and strain at break, and Young’s modulus of PLA [91] [92] . The number of extrusion cycles has a significant effect on the tensile, impact strength, and the cold crystallization temperature (Tcc) of PLA. However, there is no effect on the T_g [93] [94] . For 3D printing PLA filament, the recycled materials present similar properties to the virgin materials. As a result, tensile strength, modulus, and hardness decreased, but shear strength of recycled materials increased [95] .

Acrylonitrile butadiene styrene (ABS) is well-known as an amorphous copolymer with good mechanical properties, such as heat resistance, high rigidity, toughness, and impact strength. A hot printing bed is required for ABS with an extrusion temperature of 215˚C - 250˚C. Furthermore, ABS faces shrinkage and warping problems during printing [86] . It is a non-biodegradable and material capable of emitting toxic smoke during 3D printing. It can be pointed out that repeated extrusion cycles (up to five) significantly affect the impact strength of ABS rather than its tensile properties [18] [96] . Based on the results of Mohammed et al. [97] , the flow rate of recycled ABS filaments was relatively unchanged with increasing extrusion temperature. The print quality of recycled ABS filaments was similar to the commercial ones. However, a 13% - 49% decrease in ultimate strength was found in samples printed from recycled ABS filaments. Sharing the same point of view, Charles et al. [98] concluded that the tensile and impact strengths of recycled ABS filament were similar to virgin ABS after two recycled cycles. A slight change in polymer viscosity was observed, resulting in a small improvement in print quality. Furthermore, T_g was mostly constant during the extrusion process.

Polyethylene terephthalate (PET) is a semi-crystalline thermoplastic with an extrusion temperature of 212˚C - 235˚C. For 3D printing PET filament, a heated bed does not require. PET is fairly hard, recyclable, and odorless when printing. This material has good properties such as good tensile, impact strength, and thermal stability. However, PET is not widely used for FDM printing, because of its high melting temperature, water absorption, and low crystallinity. Currently, available PET filaments are recycled PET and glycol-modified PET (PET-G). PET-G is an amorphous plastic, where the ethylene glycol chain is replaced by cyclohexanedimethanol, leading to reduce its brittleness [99] . PET-G is durable, biocompatible, flexible, recyclable [100] , and easy to deal with during 3D printing filament compared to PET [101] . For 3D printing filament, PET-G is considered a material possessing good properties of ABS (durability, high strength) and PLA (biodegradable, high flexibility) [100] . Therefore, PET-G and recycled PET are considered an alternative to virgin PET for 3D printing. Zander et al. [102] concluded that the molecular weight and the viscosity of recycled PET filaments were unchanged after two extrusion cycles. However, it is difficult to achieve uniform diameters for filaments, due to the low viscosity of recycled PET. Furthermore, Schneevogt et al. [99] compared the potential of using recycled PET and PET-G filaments for 3D printing. They pointed out that there was a small variation in the linear region of stress-strain curve, but a significant difference in the non-linear region between PET-G and recycled PET. Specifically, ductility failure was found in PET-G, whereas recycled PET showed a tendency for brittle failure in the non-linear region. They, therefore, recommended using these materials only for engineering designs and avoiding non-linear deformation.

High-density polyethylene (HDPE) is a semi-crystalline thermoplastic with excellent properties such as high tensile strength, stiffness, a rather low melting point (~120˚C), and highly crystalline. It is a lightweight material, flexible, easy to dye and mold, non-water absorbent and chemical resistant [20] . Recycled HDPE is available for filament extrusion because many packaging products (e.g., detergent bottles and milk jugs) are made from it. Moreover, an extrusion temperature is controlled at 180˚C - 190˚C for recycled HDPE. Unlike PLA and ABS, HDPE is, however, rarely used in 3D printing filaments. High-temperature nozzle and heating bed are required for HDPE. Furthermore, poor adhesion, shrinkage/warping, as well as stress-induced deformation during printing are found in recycled HDPE [103] . However, several changes in FDM process parameters or polymer modification could improve the quality of the printed parts. A base layer or adhesion tools (raft, brim) will improve the adhesion of the printed part to the printing surface. Recycled HDPE was successfully printed for the first time for the functional boat by Washington Open Object Fabricators (WOOF) using the FDM technique. They built a unique build plate using a fused-HDPE surface. The extruder was equipped with a heater to heat the previous layer. The CAD model was then adjusted to print a sacrificial flange with the boat to avoid warping [103] [104] . Moreover, Baechler et al. [105] concluded that recycled HDPE filament could be fed consistently into a 3D printer with a constant extrusion rate.

Polypropylene (PP) is another semicrystalline thermoplastic widely used in industry and in the manufacturing of everyday objects along with HDPE, and PET. PP has good properties such as good chemical, abrasion, fatigue, and environmental stress crack resistance, shock absorbing, relative rigidity, and flexibility [106] . However, PP has a low-temperature resistance and is sensitive to UV rays, resulting in its susceptibility to thermal expansion. For 3D printing filament, recycled PP has recently gained interest due to its availability and recyclability. PP can be recycled up to four times by thermal processing without much alteration of its properties. Vidakis et al. [107] concluded that the tensile properties of PP were affected by thermal stress after six recycling cycles. In contrast, flexural behavior could be improved; impact strength and microhardness could be maintained under repeated thermal reprocessing. Recycled PP is, however, still not widely used for making 3D printing filaments due to its warping and poor interlayer adhesion issues. The significant diameter variation and elliptical shape are found in recycled PP filament. Furthermore, recycled PP presents a considerably high flow as compared to ABS and PLA [106] . Recently, Kumar et al. [108] pointed out that the ideal temperature was 210˚C - 230˚C for printing PP using the FDM technique. Atsani and Mastrisiswadi [109] optimized the extrusion process conditions such as spooler and extrusion speeds to obtain recycled PP filaments. The results showed that the rough and curved surface of filaments still exists. As mentioned earlier, the mechanical properties of recycled PP are relatively unchanged during recycling cycles. Therefore, recycled PP is a potential material for 3D printing in the future.

Polystyrene (PS) is an amorphous polymer of high clarity, hard, but rather brittle. PS can be found in the packaging and insulation applications, disposable cups, and bowls. PS is considered a difficult material to recycle due to the high cost of transport [110] . It is often not recycled locally but must be transferred to a recycling facility, leading to increased recycling costs and investment capital for companies [111] . In fact, PS foam is completely recyclable for FDM filaments production. However, the recycling rate of PS foam is still relatively low [112] . For 3D printing filament, high-impact polystyrene (HIPS) is commercially available. HIPS is similar to ABS with high impact resistance. However, it is a biodegradable material and easy to fabricate. For 3D printing filament, the extrusion temperature is 190˚C - 210˚C, and a heated printing bed is required for HIPS [78] . Recently, Ng et al. [110] compared the properties of recycled PS foam with those of HIPS for FDM filaments. As the result, 45% higher tensile strength and 52% greater stiffness were found in recycled PS compared to HIPS. Recycled PS exhibited a more brittle tendency than HIPS. Furthermore, recycled PS filament showed lower viscosity than HIPS at high temperatures. Based on the results of this study, recycled PS could be a potential material to produce 3D printing filaments. Another study on the properties of recycled and virgin PS filaments has been conducted [113] . The results showed that the fracture surface of recycled PS filaments was uniform and without defects after the tensile test. The Tensile strength and T_g of recycled PS were lower than those of virgin PS. The melt flow index (MFI) was, however, similar for both recycled and virgin PS filaments.

The fillers used for PLA-based composites are typically cellulose and natural fibers. The use of wood fibers as reinforcement in PLA polymer was developed for 3D printing filament. Kain et al. [114] concluded that the mechanical properties of wood fiber/PLA composites were improved by adding wood fibers up to 25 wt% compared to pure PLA. However, the authors pointed out that the mechanical performance was dependent on the infill orientation of fibers. Furthermore, increasing the print width reduces the cohesion of the wood fiber/PLA, resulting in a decrease in tensile strength and an increase in water absorption [31] . The effect of lignin on the thermal and mechanical properties of lignin/PLA filament was also investigated. Gkartzou et al. [115] showed that adding 5 wt% lignin makes the composites more brittle and decreases the break elongation. In addition, a reduction in tensile strength and Young’s modulus by 18% and 6%, respectively, were found compared to pure PLA. Recently, however, Long et al. [116] have successfully produced PLA composites with excellent properties for the FDM technique. Ethyl acetate treated lignin nanospheres (EALNSs) with a high specific surface and uniform size, were used to reinforce PLA. As a result, lignin nanoparticles can improve the melt flow and mechanical properties of 3D printing products. The flexural, tensile, and impact strength of EALNSs/PLA composites were increased by 130.8%, 56.1%, and 14.2%, respectively, by adding 0.5 wt% lignin nanoparticles. Recycled continuous carbon fibers from 3D printed parts were used to strengthen the material. The carbon fiber/PLA composite had 25% higher flexural strength as compared to the original printing composites [117] . However, there are still many challenges in achieving good quality of printed products through the FDM technique. Heidari-Rarani et al. [118] modified the processing conditions to obtain a reliable print such as using polyvinyl alcohol (PVA) solutions to increase the cohesion between carbon fiber/PLA, rapid cooling of printed composites, the simultaneous injection of carbon fiber and melted PLA. As a result, the tensile and bending strength of printing composites were increased by 35% and 108%, respectively, as compared to pure PLA. The result demonstrated that there was a good adhesion between the interface of carbon fiber and PLA. Contrariwise, the main failure mode of carbon fiber/PLA composite was the delamination. In addition, basalt fiber reinforced PLA filament was also studied by Yu et al. [119] . They have successfully printed basalt fiber/PLA composite via the FDM technique with a lighter weight and better mechanical properties than those of conventional molding materials. It should be noted that the voids (inter- and inner-filament voids) still exist during the 3D printing process. Fiber length and fiber orientation played a key role in the mechanical properties of composites.

For ABS-based composite, carbon fibers (CF) are considered typical filler. ABS reinforced with short carbon fibers of an average length of 150 µm was investigated for fabricating the 3D printed parts by the FDM technique [120] . The results showed that the tensile strength increased by 22.5% with 5 wt% carbon fiber as compared to the pure ABS. However, the porosity increased as the carbon fiber content was higher than 10 wt%. Yang et al. [121] also studied the continuous carbon fiber/ABS composite with carbon fiber content of 10%. The composite had greater tensile and flexural strength than neat ABS and similar properties to injection-molded ABS parts. The interlaminar shear strength, however, was only 2.8 MPa, a very small value as compared to the shear strength (24 MPa) of CF/ABS parts obtained by injection molding due to the poor interface. It should be pointed out that the use of short carbon fiber can reduce the distortion and warping of ABS [122] . Tekninalp et al. [123] also successfully printed short carbon fiber (with a length of 0.2 - 0.4 mm) reinforced ABS by the FDM technique. Sharing the same point of view, the authors reported that the tensile strength and modulus of FDM parts increased by 115% and 700%, respectively. Contrariwise, 20% of void formation was found in the composite sample due to the gaps of deposition lines and poor adhesion between carbon fibers and ABS matrix. However, the porosity of the CF/ABS composite parts can be reduced from the auxiliary heating process, by mounting an auxiliary heating plate on the printing head of the 3D printer [124] . It should be noted that this auxiliary heating should not exceed the degradation temperature of ABS. Furthermore, glass fibers (GF) were also used to reinforce ABS. Billah et al. [125] reported that the stiffness of GF/ABS composites increased by 84% compared to the neat ABS. In contrast, the composites had similar thermal stability to neat ABS. Moreover, natural fibers such as Kevlar, palm, bamboo, pine cone, and rice straw fibers are also potential fillers for ABS-based composites. The addition of Kevlar and carbon fibers to the ABS matrix improves its rigidity and ductility [126] . Filaments produced from ABS containing 15 wt% of palm fibers show 42% higher hydrogen bonding and similar T_g compared to the neat ABS [127] . The ABS matrix reinforced with chemically modified bamboo fibers shows reduced density, but the mechanical properties of the composites remain unchanged [128] . In addition, after chemical treatments by alkaline and bleaching, adding 2 - 5 wt% pine cone fibers into ABS matrix does not change the filaments diameter and density [129] . The rice straw fiber content (5 - 15 wt%) reinforced ABS was investigated by Osman et al. [130] . The results showed that the tensile and flexural strength of rice straw/ABS composites decreased as the rice straw fiber content increased, resulting in an increase in water absorption. Although natural fibers can reinforce ABS, their concentrations in composites are still low in current studies. Natural fiber/ABS composites generally present good properties with 5 wt% fiber content.

Carbon fibers are commonly used to reinforce PET for parts obtained by the FDM technique. With a carbon fiber content of 15 wt%, the elastic modulus, tensile, and shear strength of CF/PET composites increase by 180%, 230%, and 40%, respectively, compared to neat ABS [131] . As mentioned earlier, commercially available PET filaments are made from recycled PET and glycol-modified PET (PET-G). Kichloo et al. [132] revealed that adding 20 wt% of carbon fiber into PET-G matrix resulted in a maximum of 43.7% and 25% in tensile and flexural strength, respectively, for honeycomb pattern. However, carbon fiber reinforced PET-G shows an increase in melt viscosity and a weaker interlayer bonding, resulting in a reduction of its mechanical properties compared to neat PET-G printed parts [133] . The optimization of printing parameters, therefore, was conducted. Post-processing was performed when the annealing temperature was higher than the T_g of PET-G [133] . The printed temperature of 250˚C, 0.1 mm of layer height, and 0.6 mm of nozzle diameter were reported to optimize CF/PET-G printed parts [134] [135] . As a result, an increase in mechanical properties, and a void content of 3% were found. In addition, the filament properties remained unchanged when virgin PET-G was replaced with recycled PET-G with 25 wt% of carbon fiber [136] [137] . Recently, Carrete et al. [138] revealed that post-consumer textile could be used to reinforce recycled PET (rPET) matrix. Using the surface modification technique (acid hydrolysis and silane functionalization), the cotton fibers made the melt flow index (MFI) of composite higher than neat rPET. Also, a ductile fracture of cotton fiber/rPET composites was found.

As mentioned earlier, there are many problems with printing HDPE filaments such as poor adhesion, shrinkage/warping, and stress-induced deformation [139] . Very little information has been found in the literature about the 3D printing of HDPE composites by FDM. Recently, however, natural fibers (birch and wood fibers) reinforced HDPE have been discovered for fabricating 3D printed parts by the FDM technique [140] [141] . Koffi et al. [140] successfully printed birch fiber/HDPE composite by the FDM for the first time without significant warping, shrinkage, and other geometric deformation problems. The authors revealed that the composite was composed of HDPE matrix with 10 - 30 wt% of yellow birch fibers as filler and 3 wt% of maleic anhydride as a coupling agent. The results showed that shrinkage, warping, and geometric deformation of the composite were overcome. The deformation was reduced up to 80%, and Young’s modulus increased up to 35% as fiber content increased compared to neat HDPE. In addition, Migneault et al. [141] also created a potential HDPE composite for 3D printing. 40 wt% of wood fibers were used to reinforce HDPE and 3 wt% of maleated polyethylene (MAPE) was used as a coupling agent. Observing surface chemical characteristics, the results showed that the strength of wood fiber/HDPE increased as the level of carbohydrates on the fiber surface increased. Moreover, Gregor-Svetec et al. [142] studied cardboard dust/HDPE composite materials for 3D printing filament. The authors pointed out that the high porosity, structure nonuniformity, decreased crystallinity, and lowered T_g, were found in composite filaments as cardboard dust content increased. As a result, a decrease in mechanical properties, tenacity, and elastic modulus was observed when 20 wt% cardboard dust was added. Nevertheless, cardboard dust/HDPE filament could be printed when the cardboard content was up to 50 wt%.

Along with HDPE, PP is also not a typical material used for 3D printing by the FDM technique because of its warping and shrinking during the printing process. However, adding coupling agents made PP composite easily printable. Stoof and Pickering [143] pointed out that the maleated polypropylene coupling agent improved the mechanical properties of harakeke fiber (New Zealand flax)/PP composites. As a result, the tensile strength and Young’s modulus of PP composites increased by 74% and 214%, respectively, compared to neat PP. After the printing process, however, these properties tended to decrease because of the stress relaxation of polymers. In contrast, with 30 wt% harakeke fiber content, the shrinkage of composite was reduced by 84%. Sharing the same point of view, Wang et al. [144] revealed that the incorporation of maleic anhydride polypropylene (MAPP) into cellulose nanofibril (CNF)/PP composite improved its mechanical properties. The results showed that the flexural strength and modulus of compatibilized PP composite containing of 10% wt CNF were improved by 5.9% and 26.8%, respectively, compared to neat PP. Also, Spoerk et al. [145] successfully printed carbon fiber/PP composite containing MAPP coupling agent. With 10 wt% carbon fiber content, a uniform filler dispersion, a good interface adhesion, and an increase in mechanical properties were found for its composites. However, the authors revealed that the printing orientation affected the mechanical properties of the composite. In contrast, Sodeifian et al. [146] found that adding maleic anhydride polyolefin (POE-g-MA) to glass fiber/PP composites resulted in a decrease in modulus and strength and an increase in the flexibility of the composite. In addition, surface modification is also a way to improve the properties of microcrystalline cellulose (MCC)/PP composites [147] . The use of n-octyltriethoxysilane to modify the MCC improved the dispersibility of the MCC into the PP matrix. The results showed that the filaments had a good surface finish, good mechanical properties, and easy printing. Furthermore, recycled PP (rPP) has also recently been interested in composite 3D printing [148] [149] . Rice husk/rPP filament had been successfully printed with a fiber content of 5 - 10 wt% [148] . The results showed that the composite density decreased, and its crystallinity increased as rice husk fiber content increased. The 3D printed part presented lower warping compared to printed neat rPP. The water absorption of composites increased because of the hydrophilic behavior of natural fibers. However, the study found that the fracture took place at the interface between the natural fibers and the rPP matrix [149] .

As mentioned earlier, PS foam is completely recyclable to produce FDM filaments. Recently, recycled PS (rPS) from post-used expanded polystyrene foam (EPS) has been investigated to produce FDM filaments [150] . 2.5 - 10 wt% of corn husk fiber was used to reinforce rPS for 3D printing. In addition, a layer of glue was added onto the surface of the print bed to improve the first layer adhesion. The results showed that the composite filament containing 10 wt% of corn husk fiber failed to be printed. This cause was explained by the premature thermal degradation and high melt viscosity of corn husk fiber/rPS composites. In contrast, rPS containing 2.5 - 7.5 wt% of fiber could be printable. However, the tensile strength and modulus of the composite decreased as corn husk fiber content increased. The dull and rough surfaces as well as a slight decrease in thermal stability were found for corn husk fiber/rPS composites. Moreover, cellulose nanocrystal/PS composite was also considered a potential material for 3D printing. Lin and Dufresne [151] performed the surface modification of cellulose nanocrystal using polyethylene glycol/polyoxyethylene (PEG/PEO) to reinforce the PS matrix. As a result, the mechanical and barrier properties of composites were improved. Therefore, cellulose nanocrystal/PS composite could be used for 3D printing.

The difficulties encountered in printing polymers by FDM are not only explained by the high temperatures required for their transformation; the weakness of inter-layer adhesion can also lead to a significant drop in mechanical properties. A viable solution to this problem consists in reinforcing the polymer matrices by adding synthetic or natural fibers. However, while reinforcing fibers are less adhesive than thermoplastic polymers, interlayer adhesion is weakened by their presence. The mechanisms of polymer-fiber inter-layer adhesion for FDM printing are complex and the theoretical and experimental bibliography is still limited [152] . Considering that the adhesion mechanisms of polymer-polymer interfaces are now relatively well understood, polymer blends have recently been studied and successfully printed through the FDM technique.

Harris et al. [153] successfully produced PLA/PP blends with compatibilizer PE-g-MAH (polyethylene graft maleic anhydride) by the FDM technique. With 7.5 wt% PP content, the materials presented good thermal stability. Ausejo et al. [154] also found that the thermal stability of PLA/PHBV poly(3-hydroxybutyrateco-3-hydroxyvalerate) blends was improved by self-compatibilization during the degradation of materials. The thermal stability of PLA/PHBV increased as PHBV content increased. In addition, PLA can be blended with other (co)polymers for 3D printing filament. PLA/S-co-MMA poly(styrene-co-methyl methacrylate) blends were recently studied [155] . The result showed that the thermal decomposition temperature of PLA/S-co-MMA blends was lower than that of amorphous PLA and poly(S-co-MMA). In contrast, the PLA-based blends had a higher Young’s modulus than amorphous PLA. Another study also showed that PLA/ PHBV/PBAT poly(butylene adipate-co-terephthalate) blends produced by FDM printing had a three-time higher tensile strength than those of injection specimens [156] . However, poor interphase adhesion still exists for polymers blend materials. The mechanical properties of polymer blends were dependent on the applied infill orientation [157] . FDM Filament based on PLA containing up to 20 wt% of natural rubber (NR) was produced [157] . The result showed that NR improved the elongation at break and impact strength using a linear infill parallel to the length of specimens. PLA/BioPBS (poly(butylene Succinate)) biobased filament with BioPBS content higher than 50 wt% was unprintable due to the high viscosity and low thermal stability of composites [158] . The results showed that the coefficient of linear thermal expansion (CLTE) decreased as BioBPS content increased in blend filaments. However, the ductility and crystallinity of PLA were improved by adding BioPBS. The 3D printed PLA/BioPBS with 10 wt% of BioPBS presented higher tensile and impact strength than neat PLA.

ABS blends are also considered a suitable material for 3D printing filament. Monofilaments were prepared from ABS/UHMWPE (ultrahigh molecular weight polyethylene) with SEBS (styrene ethylene butadiene styrene) as a compatibilizer [159] . As a result, monofilaments were successfully printed when the content of UHMWPE was less than 25wt% in the ABS/UHMWPE blend. The smoothest surface was found in 75:25:10 ABS/UHMWPE/SEBS. Furthermore, blending ABS with TPU (thermoplastic polyurethane) was also investigated [160] . The results showed that blends containing 10 - 20 wt% of TPU had improved inter-layer adhesion without loss in yield strength. In contrast, the blends maintained a good adhesion with 30 wt% TPU, while the yield strength was closed to that of neat TPU rather than ABS.

On the other hand, thermoplastics with poor 3D printability such as HDPE and PP were also successfully printed via blending with highly printable thermoplastics such as PLA and ABS [161] . For the first time, PLA/HDPE and PLA/PP blends without additives were successfully printed by Choe et al. [161] with optimized FDM printing parameters. Moreover, microfibrillar composites (MFCs) of PP/PET blends and PP/PS blends were successfully processed by the FDM technique [162] [163] . PET and PS could be stretched into fiber form and oriented along the deposition direction during the FDM process. Both PET and PS fibers enhanced the crystalline structure of PP, resulting in the superior mechanical properties of PP/PET blends and PP/PS blends, respectively, compared with neat PP.